1. Field of the Invention
The present invention relates to image processing for image signals representing a plurality of base colors or image signals representing a combination of colors including base colors and spot colors.
2. Description of the Related Art
Recently, the digital printing technique is certainly increasing its utility value on the on-demand printing market and a printing market for small numbers of copies. In particular, full-color printing using electrophotography is superior other printing techniques in productivity, printing cost, ease of maintenance, and the like, and is quickly expanding its market. A great deal of attention is paid not only to conventional electrophotographic full-color printing using the toners of four, C, M, Y, and K base colors (called process colors), but also to multicolor printing. Multicolor printing uses the toners of spot colors having special color reproduction, such as red R, green G, blue B, orange O, and light colors, in addition to the process colors.
In inkjet printers, a printing technique using spot color inks in addition to the four base color inks has already been developed and commercialized, and many signal generation methods for this technique have been proposed. For example, Japanese Patent Laid-Open Nos. 2002-154239 and 2002-154240 disclose a technique of replacing M and C with blue (B) ink in order to reduce graininess.
As a feature of electrophotography, if the toner amount attached per unit area is too large, scattering of toner, winding of print paper around the fixing mechanism, and the like occur. These problems degrade the image quality, and may damage the apparatus. As a technique for suppressing these problems, there is proposed a method of converting a toner amount used per pixel to a predetermined value or less in accordance with the apparatus and toner characteristic. This method is disclosed in, e.g., Japanese Patent Laid-Open No. 2005-101934.
As a signal processing technique which assumes electrophotography using toners of more than four colors, for example, Japanese Patent Laid-Open No. 2004-58624 discloses a technique of generating a color separation table when the toners of six colors are used. This technique generates a six-color separation table for using light cyan c and light magenta m by separating C and M signals on the basis of a CMYK color separation table and the colorimetric value of a material actually printed within the range of a specified amount of toner used. This six-color separation table is used to control the toner consumption amount.
When toners of more than four colors are used, a signal value corresponding to the amount of toner used within a range appropriate for the apparatus and toner characteristic can be generated by applying the above-described color separation table to general image processing.
However, the printer receives print data of various formats. The printer may need to treat image data which has already been separated into signal values of more than four colors upon reception, or image data of a special format to which the six-color separation table cannot be applied.
When color separation processing prepared in advance by converting image data of a special format is applied, the total toner amount may exceed the upper limit value of the apparatus, or color reproduction performance may deteriorate. An example of image data of a special format is image data in the DeviceN color space of PostScript®. Note that the DeviceN color space is expressed by n components (n-colors). Image data separated into multiple colors by a unique definition is also image data of a special format.
When signal processing using the color materials of spot colors in inkjet printing is applied to electrophotographic full-color printing, toner images of the spot colors are formed on a transfer member in addition to C, M, Y, and K toner images. These toner images are transferred onto print paper, heated, and fixed. As the total toner amount increases along with an increase in the number of toner colors, a large load is applied to each process in the electrophotographic process.